Scientists used ultra-high-speed, high-intensity lasers to make its melting point 14 times faster, without converting solid metals into liquids.
The record-breaking experiments described in a study published in Nature on July 23rd are the first reliable way to accurately measure temperatures in extremely hot systems, breaking decades-old theories of solid stability, the researchers said.
Unusual material states, such as the plasma surrounding the sun or the high-pressure core of a planet, can reach incredible temperatures of millions of degrees Fahrenheit. However, as scientists struggle to measure short-lived, high-temperature materials quickly enough to achieve reliable results, it has proven challenging to actually place numbers on this so-called “warm dense material.”
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“There is a great technique for measuring the density and pressure of these systems, but not temperature,” said Bob Nagler, a scientist at the Department of Energy’s SLAC National Accelerator Laboratory, in a statement. “In these studies, temperature is always an estimate with large error bars and actually maintains theoretical model. It’s been a problem for decades.”
Therefore, speed was key to successful measurements. To achieve this, the team used a 45 femtosecond (45 seconds of sec) X-ray laser pulse to rapidly heat the thin gold film. As radiation passed through the crystal film, the atoms vibrated at frequencies directly related to the rise in temperature. A second pulse was fired on the hot sample, and then these oscillating atoms were scattered, and the frequency shift of these deflecting beams gave quantitative measurements of the velocity and temperature of the atoms.
However, researchers have realized they have achieved more than new measurement techniques. “We were surprised to find a temperature that was much higher than initially expected, disproveing a theory that has been around since the 1980s,” Thomas White, a co-lead research author who is an associate professor of physics at the University of Nevada, said in a statement.
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The solid gold sample reached an astounding 19,000 Kelvin (33,700 degrees Fahrenheit, or 18,700 degrees Celsius). “This is probably the hottest crystalline material ever recorded,” White added in another statement. “We were hoping that the gold would heat up quite heavily before it melted, but we didn’t expect a 14x increase in temperature!”
Typically, solids and liquids have defined temperatures that change from one state to another. However, under certain conditions, the material can be heated beyond these limits without changing its condition. This is a phenomenon known as overheating. This effect is sometimes seen in microwaved water. If the container is smooth, there is no irregularity that can form bubbles, so liquid water bypasses 212 f (100 c) without boiling. However, slight disturbances can cause “catastrophes” and when this metastable condition breaks, the water will boil explosively.
In the 1980s, physicists calculated the limits of this overheating effect of solids as three times the melting point, and were called “entropy catastrophe.” Beyond this point, solids theoretically have greater entropy or obstacles than their liquid form, breaking the second law of thermodynamics. The law states that entropy must always be increased, so the idea that solid, well-placed particles can be more obstacles than the random distribution of particles in a liquid is an impossible contradiction.
So how did the gold sample remain solid at 14 times its melting point? The team proposed that the enormous rate at which they heated the gold would prevent the crystal structure from expanding during the timescale of the experiment.
“It’s important to make it clear that we didn’t violate the second law of thermodynamics,” White said. “What we demonstrated is that if the material is heated very quickly, these catastrophes can be avoided. In our case, within a trillion.”
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